The milk activity (lactic acid production rate) of Lactococcus lactis starter cultures is a key feature that determines their use for the manufacture of cheese and other fermented dairy products. The simplest laboratory tests involve inoculation of a reconstituted skim milk medium with a strain, incubation over a cheese temperature profile and the determination of pH after a fixed time (e.g. 5 hours). Similar tests have been used to measure temperature and salt concentration effects, to monitor the effects of seasonal milk variation and starter culture manufacturing procedures, and to detect phage infections. Continuous monitoring of acid production is a preferable alternative, since it provides more information with the potential to calculate a range of kinetic parameters. In this investigation three methods for continuous monitoring of starter culture activity in milk were evaluated. The ELIT 8-channel ion/pH analyser is able to continuously monitor pH (NICO 2000 Ltd, Harrow, U.K.). The L*a*b*SMART reflectance colorimeter system monitors the L*a*b* value of milk containing a pH sensitive dye (LabSMART LLC., Logan, Utah, USA). This system uses a flexible multisample layout capable of handling multiple 96-well microplates. The HydroPlate system uses 96-well microplates with integrated pH sensors (PreSens, Precision Sensing GmbH, Regensburg, Germany). An advantage of the 96-well plate systems is the high-throughput ability to screen large numbers of strains, strain variants or strain combinations for their milk activity and phage sensitivity. However, this study showed high variation in the absolute and relative acid production rates of cultures determined by different methods, believed to be due to lack of uniform temperature and evaporation from microplate wells and oxygen inhibition of some strains due to higher culture surface:volume ratios. Use of larger culture volumes and direct pH measurement remains the method of choice for reliable comparisons of activity and predictions of industrial performance.